1. Field of the Invention
The invention relates to active systems for isolating and canceling noise and vibrations generated by machinery.
2. Description of the Related Art
Most rotating or reciprocating machines generate vibrations. The vibrations are often transmitted to the structure supporting the machine and into the surrounding air. Structural vibrations may damage nearby machinery or the structure itself. Vibrations transmitted into the air, on the other hand, are perceived as soundwaves. Depending on the source and the environment, the sound from the machine may be uncomfortable to those in the area, or even dangerous.
In the past, unwanted noise and vibration has been controlled by muffling or isolation. However, using the principle of superposition, noise and vibration can also be controlled by production of an acoustic signal having the same spectral characteristics as the unwanted noise or vibration but 180 degrees out of phase (anti-noise). Several aspects of anti-noise and its applications are discussed in an article by Professor Barrie Chaplin, entitled "Anti-Noise--The Essex Breakthrough," published in CME Magazine, January 1983, pages 41-47.
U.S. Pat. No. 4,527,282, issued to Chaplin et al. on Jul. 2, 1985, discloses a system for canceling an unwanted acoustic signal. A speaker generates a canceling acoustic signal, which is mixed with an unwanted acoustic signal. A microphone senses the residual acoustic signal, which is then amplified and inverted to drive the speaker. Systems of this type are typically prone to instabilities and tend to be effective only in a relatively restricted range of frequencies.
A system which avoids the instability problems of simple systems, such as that disclosed in U.S. Pat. No. 4,527,282, is described in U.S. Pat. No. 4,490,841 issued to Chaplin et al. on Dec. 25, 1984. In the described system, the residual signal is analyzed by means of a Fourier transformer. The resultant Fourier coefficients are then processed to produce a set of Fourier coefficients used to generate a canceling signal.
Systems which process signals in the frequency domain, e.g. using Fourier transformation, perform their function well under steady-state conditions. However, if the fundamental frequency of the noise signal changes, the system requires several cycles to re-astablish effective cancellation. This is due to the time taken to perform the Fourier transformation. If such apparatus is used in an internal combustion engine noise control system, bursts of noise will occur during acceleration and deceleration. These bursts may, in fact, have a higher peak value than the unsuppressed steady-state engine noise. Furthermore, the need to carry out high-speed digital signal processing makes these systems expensive.